This invention relates to a process and an apparatus for charging at least one liquid reactant with gas, especially in small quantities, for the production of foams, and in particular polyurethane foams. The reactant is continuously introduced into a gassing tank which is under pressure and in which a gas cushion is maintained in the upper region. Gas is absorbed from the upper region via a hollow stirrer which disperses the gas in the liquid which is situated below the stirrer in the lower region of the gassing tank. The reactant, now charged with gas, is then removed from the gassing tank for further processing.
It is well known that for the production of foams, a certain gas content, which varies according to the chemical system of the reactants, assists the cell formation. For the production of polyurethane foams from polyols and isocyanates, it is generally the polyol component which is charged with gas because the polyol generally has the higher viscosity.
It has hitherto been considered that the best way to assist the foaming reaction is to charge the component with as much gas as possible. In RIM systems (reaction injection molding) in particular, the polyol component is charged with 20 to 70 volume % of gas. A reaction mixture which is charged with gas has improved flow properties, expands more uniformly and in the mold exerts a pressure from within which prevents the formation of surface defects such as cavities in the finished molded product.
Processes and apparatus have therefore been developed for dispersing as much gas as possible in at least one of the reactants. Thus it is known, for example, (European Pat. Application 175,252; and "Kunststoffe" (1984) Number 11, pages 659/660) to charge a reactant intermittently with gas by connecting a storage tank to a small gassing vessel equipped with a hollow stirrer which absorbs gas from a gas cushion maintained in the upper region of the gassing tank. Gas is dispersed in the reactant below the gas cushion via the perforated stirrer blades. In this arrangement, the component is kept in circulation by a pumping action. The density is continuously determined as a measure of the amount of gas charged. The hollow stirrer is shut off when the density reaches the required value and is switched on again as soon as the density rises above this value. This technique is unsuitable for dispersing small quantities of gas in a component because the amount of gas introduced cannot be controlled with sufficient accuracy. The gassed component in the storage tank must be stirred to maintain homogeneity but stirring causes small gas bubbles to unite to form larger bubbles so that the component contains a wide range of sizes of bubbles. This is particularly disadvantageous for cell formation.
A continuous process has been described, a so-called on-line process (German Offenlegungsschrift No. 3,434,443), in which a gassing container is connected to the conduit leading from the storage tank to the mixing head. Here again, a hollow stirrer absorbs gas from an air cushion in the upper region of the gassing tank. This gassing tank is designed as a continuous flow tank, i.e. the component which has not yet been gassed is introduced into the lower part of the container and removed from the middle region after it has been charged with gas, but a true circulation is not maintained. In this process, the component charged with gas has contact with the gas cushion. At low gas charges, it is impossible to ensure that the gas content will be kept constant at the required level due to diffusion at the interface between the gas cushion and the gas charged reactant. Since high stirrer speeds and flow breakers are in most cases still employed for achieving high gas charges, the diffusion in these cases is even more pronounced due to the vigorously swirled up surface of the liquid reactant in the tank. This method is therefore also unsuitable for dispersing relatively small quantities of gas in a reactant.
The problem arises of finding a process and an apparatus whereby small quantities of gas, and in particular quantities below 10 volume %, based on the total volume of the gassed component, can be dispersed in at least one of the liquid reactants for the production of a foam in such a manner that the gas content is kept constant at the desired level.